a mineral that separates easily into small flat transparent pieces of rock, often used to make electrical instruments.——朗文当代大辞典

Any of a group of monoclinic minerals which are composed of hydrous aluminosilicates of sodium, potassium, magnesium, etc., and occur as minute glittering plates or scales in granite and other rocks, and as crystals with perfect basal cleavage that are readily separable into thin, transparent, heat-resistant, usually flexible laminae (which can be used as a substitute for glass, and for thermal and electrical insulation, etc.).——牛津当代大辞典  @en